1. Field of the Invention
The present invention relates to an corrosion monitoring system, which is used to provide an overall assessment of the materials degradation and the condition of protective coatings in a tank structure in which the metal is subject to corrosion, and particularly relating to a corrosion sensor for use in tanks which contain or intermittently contain conductive electrolyte.
2. Description of the Related Art
Shipboard tanks make up a significant percentage of below deck space in ships and vessels. These tanks are necessary components for the storage of liquids, for example, ballast seawater, compensated fuel/seawater, and a number of other essential liquids. The size and quantity of these tanks vary considerably for each class of ship. Each tank on a ship has a unique geometry, operational use and a set of corresponding environmental factors in which the metals and coatings are exposed. Seawater tanks, used in many ballasting operations, are subject to high salinity conditions, high humidity, the attachment of biological materials to the surfaces and repeated fill/drain cycling. Fuel tanks may be purely fuel storage or in many cases they are compensated with seawater, to minimize hull buoyancy changes as the fuel is consumed. In these compensated tanks, conditions continually vary between a petroleum-based system to that of seawater immersion. Other tanks, such as sewage (combined holding tank) and potable water, are both exposed to unique environments. Tanks are coated differently depending on usage and may or may not have galvanic anode cathodic protection, although all tanks with seawater influx are generally cathodically protected. In addition to basic usage differences, within each tank corrosion conditions and coatings performance may vary considerably. In seawater ballast tanks, areas in residual water are continually immersed in electrolyte and receive cathodic protection most of the time. The corresponding vertical wall areas and overheads undergo routine cycling during normal use and usually have wet/dry conditions along with high humidity and heat. These tanks also contain a significant percentage of structural components, which are difficult to prepare and coat effectively. Overhead coated surfaces, while often wet from condensation and high humidity, fail by effects of gravity and osmotic pressure directly at the coatings surfaces. While each of these areas are exposed to similar conditions, in general, failures for different surfaces may occur at different rates and by different mechanisms. Those tanks located on the ship exterior may additionally receive solar energy and suffer from highly variable temperature and heat cycling effects.
The maintenance of tanks is more than just re-painting the metal surfaces. Tank inspection and assessment alone requires the need for manual opening, gas freeing, staging (if necessary) and entry of trained personnel. In the U.S. Navy alone, thousands of tanks are inspected each year, with an average cost of eight to fifteen thousand dollars per tank. Each tank is typically inspected at least once every dry dock cycle, or nominally at least every 5 to 7 years depending on service or ship class. Once tanks are identified for refurbishment, U.S. Navy fleet tank maintenance costs soar to over $250 million/year. It is most cost effective to perform maintenance (staging, surface preparation, coatings application, and galvanic anode replacement) on only those tanks which are in the worst condition, especially where funds and time are limited. In order to accurately determine which tanks require maintenance, all tanks should be monitored, assessed and correctly identified for maintenance either continually or beforehand, so that the maintenance that is performed is done only when the condition of the tank preservation warrants repair.
Typically, a tank preservation system uses dielectric coatings (e.g. paint) as the primary corrosion barrier and a cathodic protection system as a secondary measure to minimize coatings degradation and to prevent galvanic corrosion of the tank material.
The cathodic protection system for a tank typically consists of a number of sacrificial anodes, typically made of a strongly electro-negative metal such as a zinc or aluminum alloy. The sacrificial anodes are often referred to as “zincs”. The sacrificial anodes are distributed through the tank and mechanically attached to the tank walls. Adequate cathodic protection is so beneficial, that in U.S. Navy ships, for example, the anode type and arrangement are defined by a Navy specification. By design, these sacrificial anodes are more “electro-negative” or “anodic” than the tank metal, commonly steel, thus creating a controlled corrosion cell where the sacrificial anode is consumed preferentially to the tank structure. Because the sacrificial anodes are selected to be more negative than most materials, they will also protect other metal components within the tank (e.g. piping, valves, cables). The protection afforded the tank metal also helps minimize premature coatings failure.
The sacrificial anodes are mechanically attached to the tank walls to prevent them from shifting during ship motions and electrically grounded to the tank walls to allow for the conduction of current from the anode to the tank. For good anode performance, anodes are generally directly mounted to the tank walls/structure. When immersed, the sacrificial anodes corrode to produce ions in the electrolyte (fluid in the tank) and correspondingly supplies electrons (current) through the metallic path to the tank surfaces. Because the sacrificial anodes supply electrons to the tank surfaces, a benign chemical reaction occurs at the tank surfaces using the electrons supplied by the anode, instead of the corrosion reaction which would occur at the tank walls if the sacrificial anodes were not present. Ideally, a sufficient number of sacrificial anodes are distributed throughout a tank, so that all areas and components within the tank are influenced by the sacrificial anodes. More sacrificial anodes may be located at the lower points within a tank with varying fluid levels, such as a ballast tank, or in areas which need more protection (e.g. near Cu—Ni piping which passes through the tank or other non-steel components). Typically, placement of the sacrificial anodes in a seawater ballast tank cathodic protection system is weighted ⅔ towards the bottom surfaces of the tank.
Even when the tank is protected by a good dielectric coating, sacrificial anodes play a significant role. No coating system is perfect, and if a coating is damaged, the exposed bare tank metal will be subjected to the tank fluid, with the exposed area being aggressively attacked and corroded. Even if the damage to the coating is small, corrosion begins, and over time, tends to undercut the intact coating around the damage thus enlarging the area of attack and damage. Coatings damage is a progressive event and a large number of small damage spots can contribute to significant damage. The installation of cathodic protection helps to prevent continued damage at bare areas and minimizes the coating deterioration and undercutting action.
Several events may happen in a tank during the time between tank maintenance. Over time, the coatings system begins to fail and more bare area is exposed. Mechanical damage plays a role, but the coating itself also adsorbs moisture slowly and moisture eventually reaches the metallic surface where corrosion begins. Imperfect or poor coating application may accelerate the moisture absorption effects or target areas which fail sooner. Whatever the failure mechanism, eventually more and more tank metal area requires cathodic protection. As demand on the sacrificial anodes increase to protect more bare area, the sacrificial anodes are consumed faster, because the sacrificial anodes are required to output increasingly greater amounts of current. Eventually, tank coatings failure occurs when the percentage of damage becomes intolerably high or when the cathodic protection system (sacrificial anodes within the tank) can no longer supply enough current with which to protect the amount of bare area.
Maintenance costs in a tank are extremely costly, because the tank requires staging, grit blasting recoating, and installation of fresh sacrificial anodes, under controlled environmental conditions and all in a very difficult non-uniform geometry. Ships with many tanks cannot repaint all tanks on a routine basis and port engineers, with highly limited resources, must decide which tanks must be recoated and when. Tank inspection is necessary in order to identify whether a tank requires maintenance. Most tank maintenance problems fall into several categories often related to the operational aspects of the ship and are roughly identified as:                a) Corrosion/structural damage.        b) Osmotic disbondment caused by condensation on overhead surfaces.        c) Coatings degradation caused by normal deterioration, variable tank levels, wet/dry cycling or depletion of cathodic protection.        d) Failure related to substandard coatings.The geometry is often unique for each tank and maintenance procedures are often complicated by many complex structural members and baffles. Working conditions within the tanks are often awkward, difficult, and potentially dangerous.        
At present, a “man-in-tank”, visual tank assessment must be performed by a trained tank coatings inspector in order to inspect the corrosion damage to the tank walls, deterioration of the coating system, and condition of the sacrificial anodes. This method of inspection is costly, time-consuming, and typically subjective in nature. Typically, visual tank inspections require that each tank be drained prior to inspection, toxic gas-freed (i.e. per OSHA/NAVOSH requirements) and subsequently certified to contain an atmosphere suitable for human entry. For each inspection, an inspector must go into the tank and visually inspect all tank surfaces and sacrificial anodes. The subjective nature of a visual inspection and difficulty in observing many areas of the tanks may result in missed areas, misinterpretation of corrosion damage, or poor assessment of general coatings deterioration.
With the economic trend toward increased time between overhauls and decreased maintenance costs, it is particularly important that tank conditions be monitored carefully, so that tanks with the greatest maintenance requirements are correctly identified. Optimally, an inspection scenario would rate all the tanks, examine the coatings degradation “trends” within the group and target those tanks within the population that are in the worst condition. Ideally, to perform this task and defray the manned inspection costs, a tank corrosion monitoring system would be available to reduce or eliminate the costly and time consuming visual inspections. The tank corrosion monitoring system could be part of a condition based maintenance plan that would monitor the coatings degradations, analyze data from tank sensors, and compare and trend the tank conditions relative to each other. Further, such a fast, inexpensive tank monitoring and inspection system would allow scarce resources to be devoted to actual tank maintenance, rather than to labor intensive visual inspection.
Because opening and preparing a tank for human entry is so expensive and time consuming, it is optimal to minimize manned inspections and best to schedule all tank repair and coating work possible within the period the tank is staged and available. Typically ship maintenance is planned months prior to arrival of the ship, requiring schedulers to either estimate tank maintenance needs based on historic tank data, or on tank inspection reports, if they are available. If tank maintenance is incorrectly scheduled, based upon inaccurate and dated human inspections, unnecessary funds may be expended to refurbish areas that do not have critical need, and other necessary-maintenance, which had been deferred in favor of the tank maintenance, may go undone.
Two major sources of data are available to the corrosion engineer concerning the condition of the tank coatings and the cathodic protection, without the need for extensive instrumentation. First, the electrochemical potential of protected steel can be measured using a standard half-cell, such as a silver/silver chloride (Ag/AgCl) reference cell, as discussed by H. H. Uhlig, “Corrosion Handbook” (1955), the disclosure of which is incorporated by reference. Where steel is protected by a zinc galvanic anode system, any bare steel surfaces and even the coated steel surfaces are polarized in an electro-negative direction forcing the steel surfaces to become cathodic, with respect to the galvanic anode. As long as sufficient anode mass is correctly located within the structure and the cathodic area requiring protection does not exceed the current capacity of the sacrificial anodes, then the surfaces will remain protected, as discussed in J. Morgan, “Corrosion Protection”, 1960, the disclosure of which is incorporated by reference. Changes in either of these states can be measured using appropriate reference half-cells installed in the tank. No convenient, long term monitoring system is available using standard half-cells, however.
Second, each galvanic (sacrificial) anode supplies electrical current as its part in protecting the metal (typically steel) structure. Measuring this level of electrical current allows a determination of how active the sacrificial anodes are, and the level of current and can be used with Faraday's law to predict anode weight loss and thus predict anode life, based on the rate of anode deterioration. A special purpose instrumented anode can be designed whereby the current output can be measured and subsequently gauged depending on the cathodic protection requirements of the tank. This special purpose sacrificial anode does not need to replace an existing sacrificial anode within the tank, but may be added to the tank in order to measure the necessary data.
A tank corrosion monitoring system that accurately monitored the coatings degradations and corrosion level which measures the current output from an instrumented sacrificial anode and measures the potential from at least one reference half cell is disclosed herein.